(1) Field of the Invention
The invention relates to a pretreatment system and method for recycling exhaust water, and more particularly to a system and method which can remove a substantial amount of silt or contamination in the exhaust water before forwarding the water into a water-purification equipment.
(2) Description of the Prior Art
For most countries, all-aspect management and reasoned allocation on domestic water resources are a crucial topic to all kinds of nations for ensuring continuing prosperity of economic development. To the authority, it is important to establish a complete package of regulations and laws, and also to carry out a solid plan of public elementary construction leading to form a complete reservation and utilization network of the domestic water resources. On the other hand, to the private institute and ordinary family, the policy for utilizing the water is to save the money without violating the laws.
As the economics grow, the need for the utility water is increased as well. In general, the conventional water-supply pattern that supplies water from the reservoirs directly into the water-consumed party is no more an if-and-only if condition that can guarantee sufficient supply of water. Especially in the district that locates intensively a lot of semi-conductor manufacturing factories or other water-consumed industries, an increasing need for stable utility water supply becomes an important issue that can never be neglected and needs to be resolved immediately.
Generally speaking, there are two measures to resolve the water shortage problem; one is to dig out more nature water resources, and another is to save the water consumption. However, in the case that the formier measure is employed, it usually implies that huge budget, laboring, and time need to be involved. Apparently, that resolving the water shortage problem by further sourcing can meet the urgent need from the industries. Also, the uncertainty for large-scaled construction is usually high and makes the work unpredictable. On the other hand, the latter measure can be carried out immediately at the consumer end, and seems to be feasible to meet short-term shortage Usually, the latter measure includes at least two resorts, cutting down the water usage and/or recycling the exhaust water.
To evaluate possible resolutions to save the water consumption, it is evident that, by cutting down the water usage, the freedom of using water is usually sacrificed and inconvenience in the normal living operation is also inevitable. On the other hand, it is believed that recycling the exhaust water is a better way nowadays to resolve the water shortage problem, especially for those industries using plenty of water. The basic idea to recycle exhaust water is to utilize a water-purifying process to revitalize the exhaust water back to an acceptable quality level compared to the water that is directly supplied from a reservoir. For industries, by recycling the exhaust water, the external water need can be greatly reduced and the manufacturing can be much controllable.
The present invention is an effort on providing a method and a system for recycling the exhaust water. The exhaust water can be the water from a texture factory, from the oceans, or from a contaminated or half-salted well. The technical fields related to the present invention include exhaust water treatment, water-purification process, and water quality-monitoring method. Following are brief introductions upon these three fields.
A. Exhaust Water Treatment
Conventionally, to improve the quality of exhaust water to a releasable level, it is usually seen that bio-treatment, chemical mixing, or active carbonating is used to de-contaminate the exhaust water before discharged to the oceans. Among these treatments, chemical mixing is the most popular method to apply, in which chemicals are used to mix with the exhaust water for forming filterable flocculent gels. Generally, chemical mixing process can be classified to a rapid mixing (or coagulation) and a slow mixing (or flocculation). The rapid mixing can have the benefit of thoroughly blending the chemicals and the exhaust water, and the slow mixing can have more processing time to allow the flocculent gels growing.
In conventional rapid mixing equipment, the reactor is an open continuous stirred tank reactor (CSTR), shaped as a cylindrical tank or a rectangular tank. In the tank, the chemicals and the exhaust water are forced to mix and form a plurality of flocculent gels. However, CSTR provides an open system that is vulnerable to the ambience. Rain, sand, or other particles can contaminate further the exhaust water in the tank. Also, due to a large volume of the tank, a dead volume that cannot be reached by the stirred flow in the tank does always exist. Conventional measures to resolve this disadvantage are to increase the stirring capacity, to modify the interior of the tank for enhancing the turbulence inside the tank, and to adjust the amount and the ingredients of the chemicals. Nevertheless, any aforesaid resort does lead to the increase of the cost.
On the other hand, a conventional plug flow reactor (PFR) provides a closed pipe flow for mixing the chemicals and the exhaust water. The PFR does not include any stirring mechanism, so that the energy consumed can be reduced. However, in the PFR, the pipe-flow mixing of the chemicals and the exhaust water is inferior to that in a CSTR; so that the PFR is seldom seen in industrial practice.
B. Water-Purification Process
The water-purification process is to filter out the minerals, bacteria, silt, and other pollutants in the water; for improving the water quality to an acceptable level compared to a drinking water. Equipment for this purpose includes an active carbon water-purification facility, a reverse osmosis system, or the like. However, ordinary water-purification equipment is usually aimed at purifying the water directly supplied from an external water pipe, not at treating the internal exhaust water. Thus, if applying conventional water-purification equipment directly to purify the exhaust water, plenty of energy and cost are usually needed for successfully completing a treating target; for the exhaust water is not the design object for such equipment to handle.
B. Water Quality-Monitoring Method
Conventionally, a silt density index (SDI) is usually provided to evaluate the water quality. SDI can be derived as follows. ##EQU1##
where t.sub.0 is the duration for a 500-ml sample water to pass a 0.45-.mu.m standard membrane, and t.sub.15 is the duration after a first 15-minute operational time for another 500-ml sample water to pass the standard membrane.
Though the SDI numbering can be effectively used to show the pollution of source water, it is incapable of showing the silt density difference among various exhaust waters. Empirically, while the sample water is filled with particles sized to 0.45-1.00 .mu.m, the standard membrane will be rapidly jammed and allow only extreme little water to pass through. As a consequence, the t.sub.15 will go extreme big, and make the SDI value reach its 6.67 upper bound. Therefore, regarding the exhaust water, the SDI is not a relevant scale to distinguish the pollution degree.
Further, the trend of the water-managing policy worldwide is led to persuade a higher recycle rate of exhaust water. For example, in Taiwan, a 1998 national conference on water resources has proposed to increase the enforced recycle rate of exhaust water to 65%, from a 31% previous rate. By raising the enforced recycle rate of exhaust water, the industry does not only need to improve the water-purification equipment, but also need to retreat any possible internal exhaust water. Hence, to provide a pretreatment system and a method for recycling exhaust water before conventional water-purification equipment is extremely important, and has obvious advantage on cost down and meeting the government's regulation.